This invention relates to novel designs for solid oxide fuel cells (hereinafter SOFC) that can convert the chemical energy of fuel and oxidant (air) directly to electrical and heat energy. More particularly, this invention relates to planar SOFCs, novel interconnectors, SOFC stacks, methods for fabricating them, and apparatus including them.
Fuel cells are electrochemical devices that convert the chemical energy of fuel and oxidant (air) directly to electrical energy and heat energy. A fuel cell consists of two electrodes, an anode and a cathode, with an electrolyte layer between them. Fuel, such as hydrogen, hydrocarbons or carbon monoxide, is continually fed to the anode and oxidized there to release electrons to an external circuit. An oxidant, such as air, is continually fed to the cathode and reduced there, accepting electrons from the anode through the external circuit. The electrolyte is a gas-tight, pure ionic conductive membrane through which only reactive ions can be transmitted
Such fuel cells have high energy conversion efficiency, since the fuel cell generates electrical energy from chemical energy directly, without any intermediate thermal and/or mechanical energy conversions. Generally a series of such cells are operated together in a stack to provide higher voltage, wherein an interconnector connects the anode of one cell to the cathode of the next cell in the stack.
Current thin film solid oxide fuel cells comprise an anode supporting electrode, a cathode electrode, and a thin film electrolyte between them.
As shown in FIG. 1, a dense, thin electrolyte layer 14 about 10 microns thick is deposited between a porous anode support layer 12 and a porous cathode layer 10. The cathode layer 10 can also be used as the supporting substrate for the thin film electrolyte layer 14.
These fuel cells have traditionally been made by depositing the electrolyte layer 14 on the anode layer 12; sintering the bi-layer at high temperatures of about 1400xc2x0 C.; screen printing the cathode layer 10 on the other side of the electrolyte layer 14; and sintering the resulting tri-layer at about 1250xc2x0 C. The need for two firing steps adds to the cost of manufacture, and, because of the difficulty of obtaining a For continuous electrode reactions. Electrons flow through the external circuit from anode to cathode, producing direct current electricity. strong bond between the cathode 10 and the sintered electrolyte layer 14, good cathode/electrolyte interface properties are not obtained.
One type of ceramic fuel cell has been made by casting a plurality of green tapes comprising an oxide powder and an organic vehicle, including plasticizers, binders, dispersants and the like, for each green tape layer, and stacking them together. The layers are laminated at a predetermined temperature and pressure to obtain a monolithic multilayer green tape stack. The stack is then sintered at high temperatures to remove the organic materials and to form a single solid body.
In order to maintain good bonding between the different layers, it is necessary to keep a high plasticity in the green tape slurry, using sufficient organic additives, typically about 10% by weight of the slurry, of binders and plasticizers. When sub-micron size ceramic powders are to be added however, more organic additives are needed, on the order of about 20-30% by weight of the slurry, in order to obtain good strength and flexibility in the green tape. However, these organic additives decrease the concentration of the ceramic particles, and increase the sintering temperature needed to obtain a fully densified layer. If a thin film layer, i.e., the electrolyte layer, is included in the multilayer structure, this traditional process has to face the challenges of handling the thin film green tape, and ensuring uniformity of the thin film layer thickness after lamination.
Different types of fuel cells are known, typically named for the electrolyte it uses. Solid oxide fuel cells use a solid ceramic as the electrolyte. Planar fuel cells use a solid, thin, flat plate ceramic as the electrolyte, which can be an oxygen ion conductor or a proton conductor. The operating temperature is above 400xc2x0 C. and generally is about 600-800xc2x0 C. with high output power density. This high temperature promotes rapid kinetics with non-precious catalyst materials, allows use of hydrocarbon fuels directly, and generates heat as a by-product. However, due to these elevated temperatures, the materials and stack design must adhere to rigorous requirements, both for the materials used and the stack design.
For example, high temperature seals are required at the edges of the electrolyte layers, which are difficult to make. Compressive seals, cement seals and glass seals have been proposed. Compressive seals, using metal rings, can lead to non-uniform stress distribution on the ceramic, causing cell cracks and unstable bonds of the cells to succeeding layers. Gas-tight cement seals are difficult to form. Glass seals are difficult to maintain and they make stack assembly difficult.
A typical thin film electrolyte can be 8 mol % yttria stabilized zirconia, hereinafter YSZ or Y8SZ. Various known methods can be used to make the dense thin film electrolyte, including tape calendering, colloid spray coating, plasma spray coating, sol-gel deposition, sputtering, dip coating, tape cast-laminating and screen printing. However, these various methods have problems of high cost, high processing temperatures and limitations on the materials from which the support anode is made.
A suitable anode can be a porous Ni-YSZ cermet about 500-2000 microns thick, to provide mechanical strength; and the cathode can be a Sr doped lanthanum manganite-(LSM)-YSZ composite about 50 microns thick. Another advantage of using Ni-Y8SZ as the supporting anode is that NiO-YSZ composite, the starting material for the Ni-Y8SZ anode, is stable with YSZ electrolyte at the high sintering temperature of 1400xc2x0 C. The Ni-Y8SZ, for the most part, is a good anode material with excellent electrical and catalytic properties.
However, the above porous support anode cannot be used with dry hydrocarbon fuels because carbon deposits rapidly in the anode at SOFC working temperatures. Further, this layer, e.g., of Ni-YSZ, must be quite thick, over 500 microns, to provide adequate mechanical support, although the effective reaction zone is a surface layer only about 10-50 microns thick. The thickness of the anode layer will slow down mass transport of fuel gases in the porous anode, and will decrease fuel utilization of the cell.
The cathode suitably can be made of a porous layer about 50 microns thick of a composite of strontium doped lanthanum manganite (LaMnSrO3) or LSM and Y8SZ.
In order to increase the voltage generated by a SOFC. a stack or series of such fuel cells is made, connected together by means of an interconnector that connects the cathode of one cell to the anode of the adjacent cell.
The interconnector materials must be electrically conductive, strong and tough at operating temperatures of 650-800xc2x0 C.; must be chemically and physically stable and non-reactive to other components of the SOFC in both oxidizing and reducing atmospheres at high operating temperatures of 600-800xc2x0 C.; must have low surface/interface electrical or ohmic resistance; and must have a TCE (thermal coefficient of expansion) that is closely matched to the ceramic components, about 10-11xc3x9710xe2x88x926/xc2x0 C. Further, air and fuel channels must be machined or otherwise formed into the interconnector. These requirements limit the choices of suitable materials from which to make the interconnectors. Conductive ceramics, such as doped lanthanum chromites, are expensive and difficult to machine. Metal interconnects corrode in the presence of reactive gases at high temperatures, weakening them structurally.
High temperature metal alloys have been tried to make interconnectors, such as nickel-based high temperature alloys. However, they have a higher TCE than other components of the SOFC stack, and they are too expensive for commercial applications.
Due to their low cost and CTE compatibility with ceramic components, Cr-containing ferric high temperature alloys, such as stainless steel, have been used as interconnector materials. However, their stability at temperatures of 650-800xc2x0 C. is not good enough because of their instability to oxygen. A plasma spray coating has been applied to these alloys in order to block oxygen from the contacting surface, but good, stable, conducting oxide protective layers are difficult to apply to grooved surfaces. Further, there is a continual slow corrosion on the fuel side of Cr-containing interconnector alloys. Thus there is a need for improved interconnector designs.
It would be highly desirable to provide a new, low temperature fabrication process for thin film cell manufacture. Novel electrode materials could be incorporated into the cell to improve fuel cell performance. It would also be highly desirable to provide a reliable interconnector and cell stack sealing method to improve cell stack performance, including long term and thermal cycling stability.
Planar SOFC stacks of the invention comprise green tape layers that have been laminated together. A three layer cell is made of a first porous electrode that supports a thin film electrolyte layer, and a second electrode on the opposite side of the electrolyte layer. The support can also be a separate layer from the first electrode. The green tapes are made from sub-micron sized ceramic particles and an organic vehicle, comprising about 3-15% by weight, and preferably about 10% by weight, of binders and plasticizers. Limiting the amount of organic additives results in a higher ceramic particle packing density in the green tape.
In accordance with the present method of manufacture, the electrolyte layer is cast onto a carrier tape, and a first electrode or electrode-support, generally the anode layer, is cast onto the electrolyte layer. The carrier tape is removed and the second or cathode electrode is deposited over the exposed electrolyte layer, as by screen printing. The resultant fuel cell layers are fired to densify the electrolyte layers and to remove the organics. As explained above, a limited amount of organic additives, including dispersants, binders and plasticizers, is used for the electrolyte layer.
By choosing solvents for the second electrode layer that only partially dissolve the binders in the electrolyte layer, and by supporting the electrolyte layer onto a multilayer stack, damage to the thin electrolyte layer is minimized and good bonding between the layers is achieved.
The resultant trilayer is then fired at about 1200-1300xc2x0 C. in air to remove the organics and densify the layers. A good interface between the electrolyte and the two electrode layers is obtained. Only a single firing step is required, at a reduced temperature.
The thin film electrolyte layers can be sintered to full density at lower temperatures because of their high particle packing density. In turn, the low sintering temperature will decrease the fabrication costs for each fuel cell, and provide more options for active material selection than higher sintering temperature materials.
Unique designs for a cell stack required for SOFC stacks, are also part of the present invention.
Sealing glass components are used at the edge of the cell and interconnector to bond a ceramic spacer on the top of the interconnector. A metal gasket is mounted on the ceramic spacer to provide a compressive seal between repeating units A flexible anode contact layer, as of metallic mesh, connects the anode with the interconnector of the next cell.
The first interconnector of the invention is made with a metal alloy base plate, a dense conductive ceramic blocking layer with air flow channels, a porous conductive ceramic layer and a glass bonding layer. The metal alloy base plate can be grooved or bonded with mesh to provide efficient fuel transport.
Still another design of the invention utilizes a metal base layer bonded to a ceramic layer. Gas and air channels are readily formed in the ceramic layer prior to firing. Contact vias are also formed integrally with the ceramic layers in known manner and filled with a conductor, as with a conductive via ink.
The present fuel cell stacks can be used to power a load, such as a rechargeable power source or an external power supply to generate power start-up.